1 Answer
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266 days before birth, all we have is a fertilized egg. (~33% chance of living birth).
The odds are calculated using data from in-vitro fertilization, and the next stage is 66%.
In in-vitro, some 50% of the eggs cannot fix to the uterine wall

262 days before birth, the egg has implanted (~66% chance of living birth)
After implantation, there is still a decent chance that the embryo will be aborted (and maybe the woman will not even know that she was pregnant).
Half of the embryos that die from here on will die because of chromosome abnormalities.

241 days before birth (~88%)

178 days before birth, entering the second trimester (~97%)
At this stage, problems are more likely to stem from the mother, not the fetus' genome. Problems with the cervix, the uterus or the placenta are (some ?) likely causes of death.

89 days before birth, entering the third trimester (~99%)
At this stage, doctors will consider the fetus a patient, and treat it if needed.